Nav: Home

Why gold-palladium alloys are better than palladium for hydrogen storage

July 09, 2018

Tokyo - Materials that absorb hydrogen are used for hydrogen storage and purification, thus serving as clean energy carriers. The best-known hydrogen absorber, palladium (Pd), can be improved by alloying with gold (Au).

New research led by The University of Tokyo Institute of Industrial Science explains for the first time how Au makes such a difference, which will be valuable for fine-tuning further improvements.

The first step in hydrogen storage is chemisorption, wherein gaseous H2 collides with Pd and adsorbs (sticks) to the surface. Secondly, the chemisorbed H atoms diffuse into the sub-surface, several nanometers deep. A recent article published in Proceedings of the National Academy of Sciences (PNAS) reports that the group focused on this slow second step, which is the bottleneck to the overall process.

In pure Pd, only around 1 in 1,000 of the H2 molecules that collide with the metal actually absorb into the interior. Hence, only these can be stored as energy carriers. However, when the Pd surface is alloyed with Au, absorption is over 40 times faster.

It is vital to get the amount of Au just right - hydrogen absorption is maximized when the number of Au atoms is slightly less than half (0.4) of a single monolayer of Pd, according to the study. This was discovered by thermal desorption spectroscopy, and by depth-measurement of the H atoms using gamma-ray emissions.

"We wanted to know what role Au plays," study first author Kazuhiro Namba says. "The Au atoms are mostly at the alloy surface. However, our results showed that hydrogen storage is improved even below this depth, in pure Pd. Therefore, Au must be accelerating the diffusion of hydrogen into the sub-surface, rather than improving its solubility."

This diffusion acts like a typical chemical reaction - its rate is determined by the energy barrier, i.e. the hurdle that the H atoms must overcome to penetrate Pd. The barrier height is the gap between the energies of the chemisorbed H atoms and the transition state they must pass through to reach the first sub-surface site.

According to density functional theory (DFT) calculations, the Au atoms destabilize chemisorbed hydrogen, thus increasing their energy and reducing the barrier. By making the surface a less stable environment for H atoms, this encourages them to penetrate more quickly into deeper sites, instead of lingering at the surface. Photoemission spectroscopy suggests that Au atoms push the energy of the Pd electrons downward, weakening their ability to chemisorb hydrogen.

However, the weakly chemisorbed H atoms are also more likely to simply desorb from the surface; i.e., return to the gas phase. This unwanted process explains why hydrogen storage is maximized with just 0.4 monolayers of Au - if any more Au is added, the desorption of hydrogen outpaces its diffusion into Pd.

"Our study reveals, at the electronic level, how Au alloying controls hydrogen absorption," co-author Shohei Ogura says. "This will help us to design better hydrogen storage materials, which will play a role in carbon-neutral energy transport, as well as solid catalysts for chemical reactions, which often depend on surface-bound hydrogen."
The article, "Acceleration of hydrogen absorption by palladium through surface alloying with gold," was published in Proceedings of the National Academy of Sciences at DOI:10.1073/pnas.1800412115.

About Institute of Industrial Science (IIS), the University of Tokyo

Institute of Industrial Science (IIS), the University of Tokyo is one of the largest university-attached research institutes in Japan.

More than 120 research laboratories, each headed by a faculty member, comprise IIS, with more than 1,000 members including approximately 300 staff and 700 students actively engaged in education and research. Our activities cover almost all the areas of engineering disciplines. Since its foundation in 1949, IIS has worked to bridge the huge gaps that exist between academic disciplines and real-world applications.

Institute of Industrial Science, The University of Tokyo

Related Hydrogen Articles:

Paving the way for hydrogen fuel cells
The hype around hydrogen fuel cells has died down, but scientists have continued to pursue new technologies that could enable such devices to gain a firmer foothold.
Keeping the hydrogen coming
A coating of molybdenum improves the efficiency of catalysts for producing hydrogen.
Hydrogen bonds directly detected for the first time
For the first time, scientists have succeeded in studying the strength of hydrogen bonds in a single molecule using an atomic force microscope.
Argon is not the 'dope' for metallic hydrogen
Hydrogen is both the simplest and the most-abundant element in the universe, so studying it can teach scientists about the essence of matter.
Metallic hydrogen, once theory, becomes reality
Nearly a century after it was theorized, Harvard scientists have succeeded in creating metallic hydrogen.
More Hydrogen News and Hydrogen Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Do animals grieve? Do they have language or consciousness? For a long time, scientists resisted the urge to look for human qualities in animals. This hour, TED speakers explore how that is changing. Guests include biological anthropologist Barbara King, dolphin researcher Denise Herzing, primatologist Frans de Waal, and ecologist Carl Safina.
Now Playing: Science for the People

#534 Bacteria are Coming for Your OJ
What makes breakfast, breakfast? Well, according to every movie and TV show we've ever seen, a big glass of orange juice is basically required. But our morning grapefruit might be in danger. Why? Citrus greening, a bacteria carried by a bug, has infected 90% of the citrus groves in Florida. It's coming for your OJ. We'll talk with University of Maryland plant virologist Anne Simon about ways to stop the citrus killer, and with science writer and journalist Maryn McKenna about why throwing antibiotics at the problem is probably not the solution. Related links: A Review of the Citrus Greening...